1. Field of the Invention
The present invention relates to an intake air control apparatus for an engine with a rotational angle detection sensor for detecting the rotational angle of a throttle valve that adjusts the degree of opening in an intake passage.
2. Description of the Related Art
In a known intake air control apparatus for an conventional engine, there has been used a flux density detection type sensor provided with a Hall element as a sensor for detecting the rotational angle of a throttle valve fixedly secured to a shaft.
That is, a measurement target having a magnetic circuit constructed by a permanent magnet and a magnetic member is mounted on a sector-shaped final spur gear which is fixedly secured to the shaft. The Hall element is embedded in a cover which is spaced from the final spur gear on a central axis thereof. The Hall element detects the rotational angle of the throttle valve by detecting a change in the density of magnetic flux lines passing through the Hall element through the rotation of the measurement target operatively connected with the final spur gear (for example, see a first patent document: Japanese patent laid-open No. 2001-289610, FIG. 2).
In the above-mentioned intake air control apparatus for an engine, the flux density detection type sensor provided with the Hall element is used so as to detect the rotational angle of the throttle valve. In this case, however, the permanent magnet is arranged on the cylindrical magnetic member, and the sensor detects a change in the flux density based on a change in the direction of the magnetic flux with respect to the measurement target. In this case, if the value of composition of flux density vectors passing through the sensor varies, there takes place a variation in the sensor output. In order to avoid this, it is necessary to stabilize the value of composition of the flux density vectors passing through the sensor in a detection angular range, and hence it is necessary to suppress a variation in the positional accuracy of the measurement target and the sensor (e.g., in the axial direction and in the rotational direction of the shaft) as much as possible. As a result, there is a problem that high mounting accuracy is required.
In particular, in recent years, both the final spur gear and the cover are often made of resin for the purpose of reducing the weight and cost of parts. Therefore, there is another problem that it is also necessary to mount these elements while taking into consideration influences due to dimensional changes of the resin according to atmospheric temperature changes and water absorption.
In addition, there is a further problem that if the sensor output becomes unstable (insufficient linearity with hysteresis), the behavior of the throttle valve is unstabilized with respect to a control signal from an engine control unit (hereinafter referred to as an ECU), so there might be caused inconveniences such as an amount of intake air as required being not able to be obtained.
Particularly, required control accuracy is recently becoming higher and higher in order to improve fuel consumption, driveability, etc., and hence the above-mentioned problems are drawing ever greater attention.